remove logging.debug output that may contain PII for production push

[jeffbl]

Until #912 is fixed, we do not want to risk PII going into logs. This removes several logging.debug lines from content-categoriser and graphic-caption preprocessors, to be reinstated once #912 is completed.

Tested on several photos to make sure nothing else appears to be leaking, and that commenting out these lines didn't introduce any issues.

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